Depends what your trying to achieve and how you like to play the game.
Personally I would train him up for 2-3 months until hes 17, with focus on key stats. Provided his training is decent and your hitting the key stats required to increase the rating number he should hit a 24/25 rating at some point during his 17th year and will therefore attain Wonderkid status. Depends what the markets like then but you can likely then sell for 1-1.5 Million C.